1. Field of the Invention
The present invention relates to an optical receiving device used in a field of optical communication technology. In “Description of the Related Art” and “Description of the Preferred Embodiments” provided in the followings, as an example of such an optical receiving device, a burst signal optical receiver for receiving light signals from each subscriber in a PON (Passive Optical Network) system and the like will be described.
2. Description of the Related Art
In the ATM-PON (Asynchronous Transfer Mode based Passive Optical Network) as an economical high-speed broadband optical access system of next generation, the Optical Line Terminal (OLT) for storing a plurality of users on the network side and the Optical Network Unit (ONU) for terminating the subscriber lines on the user side are connected via optical fiber cables, and bi-directional transmission of signals are achieved by single-conductor fiber of 1.3 μm/1.5 μm wavelength multiplexing or double-conductor fiber of 1.3 μm wavelength.
The signal from the OLT is branched by an optical branching device and transmitted to the ONU provided on the user side. Also, control signals and the like are transmitted in the outgoing direction from the ONU to the OLT. Especially, in the outgoing direction, the signals are the burst signals in which signals are transmitted intermittently so that it is necessary for the optical receiver on the OLT side to be capable of receiving the burst signals.
Standardization of the ATM-PON is proposed by FSAN (Full Service Access Networks). For example, in the FSAN Class-B standard, an optical receiver is required to operate stably against the burst signals with the extinction ratio (or optical quenching; the level ratio of “0” level and “1” level) of 10 or less while keeping a wide dynamic range of more than 500 times as much, that is, the optical input current of 630 nAp-p to 320 μAp-p. In other words, it is necessary to have a sufficient bearing capacity for changes in the “0” level since the “0” level changes according to the changes (variation) in the optical input current. Further, the extinction ratio is set to be 10 or less so as to flow a certain amount of current (prebias current) to a laser diode even in the “0” level state (extinct) so that the state of the laser diode immediately reaches the “1” level (light:optical output) from the “0” level (extinct:no optical output )
FIG. 5 shows a circuit diagram showing a first conventional example of a burst signal optical receiver. In the followings, description will be provided with reference to FIG. 5.
In a conventional burst signal optical receiver 70, a preamplifier 71 is provided with an amplitude limiting function as a means for keeping a wide dynamic range. In other words, the preamplifier 71 comprises an amplifier 12 for converting signal current Ii outputted from a light receiving element 11 into a signal voltage Vo, a feedback resistor 14 connected between the I/O terminals of the amplifier 12, and a diode 72 which is connected in parallel to the feedback resistor 14. The amplifier 12, since its input impedance is substantially infinite and output impedance is substantially zero, operates as an inverting (or reversing) amplifier having an amplification factor corresponding to the resistance value of the feedback resistor 14.
The signal current Ii outputted from the light receiving element 11 is converted into the signal voltage Vo by the amplifier 12. More specifically the signal current Ii is inversion-amplified by the amplification factor corresponding to the resistance value of the feedback resistor 14 to be converted to the signal voltage Vo. At this time, the larger the amplification of the signal current Ii is, the larger the voltage drop in the feedback resistor 14 becomes. When the voltage drop in the feedback resistor 14 exceeds the built-in voltage of the diode 72, the diode 72 is on state. As described, by connecting the diode 72 in parallel to the feedback resistor 14 of the preamplifier 71, the amplitude of the signals can be limited to a specific value or less (see FIGS. 6[a] and 6[b]).
However, the FSAN standard allows the extinction ratio=10, which is the level ratio of the “1” level and “0” level. Thus, there faces a problem when, for example, a large signal of 10 times as much as the normal amount or more is inputted, the “0” level exceeds the amplification limit value so that all the output of the preamplifier 71 is fixed to the “1” level (see FIGS. 7[c] and 7[d]). In other words, it is not possible for the related art to keep the bearing capacity against the changes in the “0” level.
In order to overcome this problem, Japanese Patent Application Laid-open No. 2000-252775 discloses a burst signal optical receiver which can effectively suppress the waveform distortion even when a light signal with a large power level is inputted. FIG. 8 is a circuit diagram showing a burst signal optical receiver 100 (a second conventional example). In the followings, description will be provided with reference to FIG. 8.
A photodiode 101 receives a light signal L and converts it to a current signal Iin. A current mirror circuit 102 comprises PNP transistors TR1 and TR2. With the current mirror circuit 102, a collector current Imon of the PNP transistor TR2 is to be always equal to a collector current of the PNP transistor TR1, that is, the current signal Iin.
A transform impedance amplifier 103 converts the current signal Iin received and obtained in the photodiode 101 into the voltage signal. In between the input terminal and the output terminal of the transform impedance amplifier 103, a feedback resistor Rf in which a resistor R1 and a MOS transistor TR3 are connected in parallel is provided. The feedback resistor Rf is for determining the gain of the transform impedance amplifier 103 and the resistance value is determined by the parallel resistance of the resistance in between the drain and the source of the MOS transistor TR3 and the resistor R1. An inverting amplifier 104 inverts the output signal of the transform impedance amplifier 103 and outputs it as a voltage signal Vout to the outside.
A transform impedance amplifier 105 converts the collector current Imon of the PNP transistor TR2 into a voltage signal. A feedback resistor R2 is for determining the gain of the transform impedance amplifier 105 and the resistance value is set to be smaller than that of the resistor R1. An inverting amplifier 106 inversion-amplifies the output signal of the transform impedance amplifier 105.
A peak detection circuit 107 detects a peak value Vpd of the output signal of the inverting amplifier 106. A reset signal Reset is inputted to the peak detection circuit 107 from the outside so that the peak value Vpd is reset every time the burst signal ends. An operational amplifier 108 outputs a signal Vg to the gate of the MOS transistor TR3 according to the level of the peak value Vpd thereby to control the gate voltage of the transistor TR3.
Next, operation of the burst signal optical receiver will be described.
First, the photodiode 101 receives the light signal L and converts it to the current signal Iin. As a result, the current signal Iin becomes the collector current of the PNP transistor TR1 so that, in the collector of the other PNP transistor TR2 in the current mirror circuit 102, the monitor current Imon equal to the current signal Iin flows. The transform impedance amplifier 105 converts the monitor current Imon from the current mirror circuit 102 into the voltage signal. At this time, the polarity of the waveform outputted from the transform impedance amplifier 105 is inverted with respect to that of the waveform of the monitor current Imon inputted to the transform impedance amplifier 105. The gain of the transform impedance amplifier 105 defined by the resistance value of the feedback resistor R2 is set to such value that the output signal is not saturated even when a large signal is inputted. Thereby, the transform impedance amplifier 105 linearly amplifies the monitor current according to the amplitude of the monitor current Imon.
The inverting amplifier 106 inverts the output signal of the transform impedance amplifier 105. The peak detection circuit 107 detects the peak value Vpd of the signal outputted from the transform impedance amplifier 105 within 1 bit, and holds the peak value Vpd in the burst section defined by the reset signal Reset. The peak value Vpd held thereby is initialized by the reset signal Reset by every burst period.
An operational amplifier 108 inverts the output signal of the peak detection circuit 107 to the gate voltage Vg and supplies the gate voltage Vg to the gate of the MOS transistor TR3 composing the feedback resistor Rf. Thereby, the resistance value of the feedback resistor Rf becomes smaller as the gate voltage Vg increases and becomes larger as the gate voltage Vg decreases. The gate voltage Vg is obtained by inverting the voltage of the peak value Vpd and the peak value Vpd represents the amount of the monitor current Imon, that is, the current signal Iin. Therefore, the value of the feedback resistor Rf becomes smaller as the current signal Iin increases and, inversely, becomes larger as the current signal Iin decreases. Also, the resistance value of the feedback resistor Rf is initialized to the maximum value when the reset signal Reset is inputted.
On the other hand, the polarity of waveform outputted from the transform impedance amplifier 103 is inverted with respect to that of the outputted waveform of the current signal Iin, which is to be inputted to the transform impedance amplifier 103, as in the case of the waveform outputted from the transform impedance amplifier 105. When a large signal of current signal Iin is inputted to the transform impedance amplifier 103, the transform impedance amplifier 103 receives the current signal Iin under the maximum gain state so that the first bit signal reaches the saturated state. However, the large signal of current signal Iin is reflected to the monitor current Imon and is controlled so that the resistance value of the feedback resistor Rf immediately decreases. Therefore, normal output waveform can be achieved after the second bit on. Also, when a small signal of current signal Iin is inputted to the transform impedance amplifier 103, the small signal is reflected to the monitor current Imon and is controlled so that the resistance value of the feedback resistor Rf immediately increases (to the maximum value). Therefore, normal waveform is outputted from the first bit on. An inverting amplifier 104 inverts the output signal of the transform impedance amplifier 103 and outputs the signal voltage Vout.
As described, with the burst signal optical receiver 100, the gain of the transform impedance amplifier 103 for inputting the signal current Iin can be changed according to the power level of the light signal to be received. Therefore, even if the power level of the light signal L becomes large, a signal sequence without distortion can be obtained. On the other hand, even if the power level of the light signal L becomes small, a signal sequence with the necessary amplitude can be obtained.
However, the second conventional example faces the following problems.
(1) Let's discuss about a plurality of burst signal optical receiver. Each burst signal optical receiver 100 has a different property in regards to the relation between the voltage in between the gate/source and the resistance between the drain/source in the MOS transistor TR3 composing the feedback resistor Rf due to the property of the individual MOS transistor TR3. Thus, there may be cases where the gain change cannot precisely follow the change in the power of the light signal L thereby narrowing the dynamic range.
(2) In general, the input terminal voltage of the transform impedance amplifier 103 requires 0.8 V or more. Also, roughly 2 V of voltage is required for operating the photodiode 101. Further, 0.8 V or more is required for operating the current mirror circuit 102. Therefore, by totaling the voltages, the supply voltage Vcc mounts to 3.6 V or more. Thus, it is not possible to operate the burst signal optical receiver 100 by 3.3 V (TYP.) power supply, which is the main stream in the optical transmission products.
(3) The current mirror circuit 102 has a complicated configuration compared to that of the photodiode 101 so that it operates considerably slow. Thus, it is hard to achieve the operation of 155.52 M bit/sec, which is the optical transmission rate of the ATM-PON system, by the current mirror circuit 102.